So what is MVVM, exactly? Instead of focusing on the historical context of where MVVM came from, let’s take a look at what a typical iOS app looks like and derive MVVM from there:
Here we see a typical MVC setup. Models represent data, views represent user interfaces, and view controllers mediate the interactions between the two of them. Cool.
In typical MVC applications, a lot of logic gets placed in the view controller. Some of it belongs in the view controller, sure, but a lot of it is what’s called ‘presentation logic,’ in MVVM terms – things like transforming values from the model into something the view can present, like taking an NSDate and turning it into a formatted NSString.
We’re missing something from our diagram. Something where we can place all of that presentation logic. We’re going to call this the 'view model’ – it will sit between the view/controller and the model:
This diagram accurately describes what MVVM is: an augmented version of MVC where we formally connect the view and controller, and move presentation logic out of the controller and into a new object, the view model. MVVM sounds complicated, but it’s essentially a dressed-up version of the MVC architecture that you’re already familiar with.
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